Every year, millions of holes are drilled into composite materials by the aircraft industry in order to join various structures together with fasteners. To join these structures efficiently, it is desirable to initially select a properly sized fastener. If the fastener that is selected is too long, then the fastener will need to be ground down, which will result in additional work. On the other hand, if the fastener length that is selected is too short, the fastener must be removed and replaced with a fastener of the appropriate length. Due to the nature of composite materials, the thickness of the materials to be joined often varies significantly from the nominal design size of the materials. Thus, in order to select the proper fastener length, it is desirable to know the approximate depth of the hole that has been created in the composite materials that are to be joined by the fastener.
A method of estimating the depth of the above-mentioned hole involves the use of feeler gages. These feeler gages can be manual or automatic. Although each of these feeler gages can adequately estimate the depth of the hole, both have undesirable limitations. For manual feeler gages, the undesirable feature is that they are inefficient because they require an operator to make a time consuming break in the manufacturing flow in order to measure the depth of the hole. This break in the manufacturing flow can be reduced by the use of automatic feeler gages that are attached to the robots that create the holes in the composite materials. Automatic feeler gages, however, are also not ideal because these feeler gages are susceptible to being broken during operation since they must physically contact the workpiece. Accordingly, it would be desirable to measure the depth of the hole using a process that does not involve physical contact with the hole.